(1) Field of the Invention
This invention relates to generation of high power radiation and more particularly to a continuously tunable free electron laser operating in the millimeter-submillimeter spectral range.
(2) Background of the Invention
Coherent radiation sources in the millimeter-submillimeter spectral range are typically molecular lasers which operate at fixed frequencies. A more recent development is that of a free electron laser (FEL). A description of this device is given in an article by D. A. G. Deacon et al, Physical Review Letters, Vol. 38, pp. 892-894, (April 1977) which is incorporated herein by reference. The free electron laser (FEL) of Deacon et al is driven by a low current beam from a linear accelerator and has limited peak power. It is best adapted to operate only in the near infrared spectrum.
Another device using the FEL principle is disclosed in U.S. Pat. No. 3,958,189 issued to Philip A. Sprangle et al which is also incorporated herein by reference. However, it requires a powerful electromagnetic pump wave to achieve amplification and is therefore difficult to realize.
Another adaptation of the FEL device is based upon a paper by D. B. McDermott et al in Physical Review Letters, Vol. 41, pp. 1368-1371, (1978) which is described and claimed in the patent application which matured into U.S. Pat. No. 4,331,936 to Schlesinger et al on May 25, 1982. Both above-identified McDermott et al article and U.S. Pat. No. 4,331,936 are incorporated herein by reference. However, the Schlesinger device utilizes a high gain interaction between the light wave and the electron beam using the principle of stimulated Raman Backscattering.
Another device for generating high power pulses of coherent electromagnetic radiation is called a gyrotron which is a device well-adapted to efficient generation of coherent powers in the wavelength range of a few centimeters to a few millimeter. The gyratron employs a beam of moderately energetic electrons which spiral in a magnetic field and interact with the radiation at their cyclotron frequency in a single-or multimode resonant cavity. Reliable operation of the gyrotron at wavelength of a millimeter or so is however very difficult as it requires high magnetic field and very stable resonator properties.
Still another source of coherent electro-magnetic radiation in this range by Bekefi et al (Coherent Radiation from a Relativistic Electron Beam in a Longitudinal, Periodic Magnetic Field by W. A. McMullin and G. Bekefi, Applied Physics Letters, Vol. 39(10), pp. 845-847 (1981) which is incorporated herein by reference) uses a beam of gyrating electrons which is compressed to its paraxial position by means of a solenoidal magnetic field around the waveguide. This electron beam is then subjected to a transverse motion in an interaction region by a longitudinal ripple (wiggler) magnetic field which is generated by a periodic assembly of rings magnetized in the axial direction. The electron beam during its accelerated motion generates or amplifies coherent electromagnetic radiation in the millimeter or submillimeter range. However, the requirement that the electron beam be adiabatically compressed along the axial position imposes stringent requirements on the performance of this device. It is thus desirable to have a device which will generate coherent electro-magnetic radiation in the submillimeter-millimeter range which is tunable continuously and which provides a flexibility of design so as to the strength of the electron beam and the size of the drift tube.